New addition: Patricia Hodge has been confirmed to play Bertie Pelham's demanding mother

The series six finale saw Lady Mary (Michelle Dockery) ruin her sister Edith's engagement to Bertie by telling him about her illegitimate daughter Marigold, and in the Christmas special she endeavours to build bridges while the secret continues to pose a threat.

Meanwhile, Mary's new husband Henry Talbot (Matthew Goode) struggles to find his place at Downton and butler Mr. Carson (Jim Carter) faces some personal challenges which prove even his is not invulnerable to change.

Producers have teased that the last ever episode will offer all the love and loss, happiness and heartbreak that fans of the show have come to expect, as the family and servants prepare to ring in the year 1926 with an unforgettable New Year's Eve together.

She's been away: The Lady Rose actress (left, pictured on her return) has been away to focus on her career

Big star: The actress found great success in the live-action retelling of the classic fairy tale

Filming of the upcoming special took place over a week at Alnwick Castle in Northumberland - which doubled as Bertie's Brancaster home - and at the iconic Ritz hotel in London.

In addition to new and returning cast members, the episode will also feature regular stars including Maggie Smith, Hugh Bonneville, Elizabeth McGovern, Phyllis Logan and Joanne Froggatt among others.

Lily previously revealed that she will make a one-off return after a year's absence in which she rocketed to fame in Cinderella and the forthcoming Pride and Prejudice and Zombies.

Her character left the show in last year's Christmas special, following her wedding to Atticus Aldridge, with the pair heading to New York together.

Welcome back: Mr. Carson (Jim Carter), Lord Grantham and the gang were pictured back on set

Man of the house: Lord Grantham will help his family ring in the year 1926

Newlywed: Lady Mary is still wearing her bridal whites after her summer wedding

Returning to filming earlier this year, the actress told Harper's Bazaar that her return to the show was nostalgic.

She said: 'It was amazing going back after a break. You are just totally re-inspired by the wonder of it, that grand house against grey skies.'

Lily went on to say: 'I have such happy, positive memories and a sense of accomplishment about being on the show. I don’t know if I’d be sitting here if it weren’t for Downton. It would have all been very different.'

Viewers captured a glimpse of Lady Rose as part of a letter that was read by Lady Mary, though the latest pictures appear to put paid to the suggestion that her character will be pregnant.

Downton Abbey returns to ITV on Christmas day.

Puppy love: Can we expect to see another new addition to the household in canine form?

The other half: Mary's new husband Henry Talbot (Matthew Goode) may struggle to find his place at Downton